Resistance heater unit for thermal overload devices and method of making the same



1949- I... D. DRUGMAND ET AL. 2,479,914

RESISTANCE HEATER UNIT FOR THERMAL OVERLOAD DEVICES AND METHODS OF MAKING THE SAME Filed on. 17, 1945 l6 IZ, 40 54 55 I9 4/ a9 5.2 54 53 55 4 I l 4/ 57 I M 5/ /40 l I I l I /"-58 i '42 44 a5 a a4 47 as l as 5/ I7 55 a7 54 HH ll 36 Patented Aug. 123, i943 @NETE hiAEES PAT Edldfiid EEEQE RESISTANQE EEATER UNIT FOR THERMAL OVERLOAD DEVICES AND METHOD OF MAKING THE SAME of Delaware Application ()ctobcr 17, 1945, Serial No. 622,750

(Cl. ZINE-76) 18 Claims. It

This invention relates to improvements in resistance heater units for thermal overload devices and methods of making the same.

A primary object of the invention is to provide a resistance heater unit which will function in the desired manner to effect tripping of a thermally responsive circuit controlling device under predetermined overload conditions, while remaining substantially indestructible under extremely high overload conditions, as, for instance, a short circuit.

Another object is to provide such a unit wherein the resistance element itself is molded from a mixture of conducting and. non-conducting ingredients in powder form in predetermined proportions, according to the operating characteristics and normal rating desired for the unit.

Another object is to provide such a unit which is interchangeable with and adapted to act as an improved substitute for the wire heater coils heretofore employed.

Another and more specific oblect is to provide a resistance heater element in the form of an, annular disk which is molded in situ between a pair of inner and outer tubular metal members, whereby the latter are held in properly assembled relation.

Another object is to provide novel means for affording a good electrical and mechanical interlocking connection between the resistance heater element and said tubular metal members as an incident to the molding operation.

Another object is to provide a novel form of wiring terminals and novel means for attaching the same to the tubular members to act as a support for the unit as a whole.

Another object is to provide novel methods of making resistance heater units of the aforementioned character.

Other objects and advantages of the invention will hereinafter appear.

The accompanying drawings illustrate certain embodiments of the invention which will now be described, it being understood that the embodiments illustrated are susceptible of modification in respect of certain structural details thereof without departing from the spirit of our invention or the scope of the appended claims.

In the drawings,

Figure 1 is a top plan view of a resistance heater unit constructed in accordance with our invention; the same being shown applied to a known form of electrical circuit-controlling device as a substitute for the usual resistance Wire heater coil.

Fig. 2 is a sectional view, on the line 2--2 of Fig. 1; the parts of the resistance heater unit being shown in elevation.

Fig. 8 is an enlarged sectional view (about three times normal scale), showing the parts of the resistance heater unit in assembled relationship to each other, and in assembled relationship to a known form of thermally responsive latchtripping device of the so-called eutectic alloy or self-soldering and Fig. 4-. is a sectional view similar to Fig. 3, showing a slightly modified form of the inner tubular member and the molded resistance heater element, but with portions of the respective terminal members broken away.

The resistance heater unit herein disclosed is adapted for use with any well-known type of electro-thermally responsive overload circuit breakers; as, for instance, an overload circuit breaker oi the character disclosed in Patent No. 2,261,632, granted November 4, 1941, to T. F. Rosing and W. C. Stevens, for Electric circuit control mechanism, and assigned to the same assignee as the present application. The device herein disclosed would be substituted for the resistance wire heater coil (shown at 98 in Fig. 12) of said Patent No. 2,261,632. As is well known by those skilled in this art, resistance wire heater coils of the character disclosed in Patent No. 2,261,632 are satisfactory except under conditions of excessive overload, under which conditions they are very likely to fuse or burn out, thus producing a fire hazard. Even though the aforementioned resistance wire heater coils may in some instances withstand fairly high overloads, it is invariably found that such high overloads have a deleterious efiect upon the resistance value or calibration oi the coil; such high overloads in most cases causing a substantial drop in the resistance value of the heater coil. The undesirability of such a result is obvious.

We have found that resistance heater units produced in accordance with our invention have the very desirable characteristic that the normal resistance value and current rating thereof when completed will be substantially maintained both under the usual overload conditions encountered in practice, and under conditions of excessive overload, such as those incident to a short cir- 3 and by properly controlling the molding pressure and the heat treatment to which the molded I article is subjected.

and l3, and the diametrically opposed side wall portions and the bottom wall of recess being also countersunk at l4, l9 and I9, respectively, to accommodate an approximately U-shaped metal yoke member l1, whose outwardly bent end portions l3 and I9 are perforated (as shown in Fig. 2) to provide clearance for the shanks of a pair of so-called self-tapping metal screws 29; said shanks being formed in any well known manner to provide for tapping thereby of suitable recesses 2|, 22 opening to said countersinks l2 and I3. The inner portion of yoke member I1 is provided centrally thereof with a circular opening surrounded by an upstanding annular flange 23.

A tubular metal member, having a bore 24 of circular cross section extending therethrough, includes a relatively long thin-walled portion 25, a relatively thick-walled portion 26, and an annular flange 21. As shown at 28 in Fig. 2 said portion 29 is longitudinally milled (or otherwise roughened in any suitable manner); the same being readily insertable within the upstanding flange 23, and the latter being then pressed or spun inwardly against the milled periphery of portion 29 to interlock said parts against rotary or longitudinal movement relatively to each other. As best illustrated in Fig. 3 the parts last mentioned are so assembled that the flange 21 will abut against the lower surface of the adjacent portion of metal support member l1.

Insertable upwardly into the bore 24 is a machined metal member having a cylindrical upper end portion 29, the upper end of which is adapted to be positioned in a substantially flush relationship to the upper end of portion 25 of the aforementioned tubular member; the outside diameter of said portion 29 being somewhat less than the inside diameter of bore 24 to insure that the former will be freely rotatable within the latter. However, a relatively thin or capillary fllm of eutectic alloy or solder 39 is interposed between the portions 29 and 29 to normally positively lock the former against rotation with respect to the latter. Adjacent portion 29 is a portion 3| of substantially reduced diameter, and the aforementioned portion 25 is preferably provided with a pair of diametrically opposed openings 32 and 33 at the line of junction of portions 29 and 3|, the purpose of these openings being well known to those skilled in the art.

Adjacent the lower end of portion 3| is another portion 34 which is freely rotatable within bore 24. However, the portion 34 and the bore 24 are provided with transversely alined peripheral grooves of segmental form in transverse cross section, which are jointly adapted to accommodate a split spring ring 35, which looks these parts against accidental longitudinal displacement, while permitting relative rotation thereof upon fusing or melting of the aforementioned solder fllm 39. Adjacent portion 34 is a flange portion 33, which is slightly spaced from the lower surface of flange 21 to afford rotary clearance between these parts. Below flange 39 is 2 portion of reduced diameter (not shown) upon which a perforated ratchet wheel 31 is adapted to be strung. The lower end of said last mentioned reduced portion is tightly riveted or upset over the lower end of ratchet wheel 31, as indicated at 39 in Figs. 2 and 3, to secure said parts against rotation relatively to each other. The numeral 39 in Fig. 2 designates a fragment of a resilient pawl carried by a movable member of suitable form not shown). which member is normally biased to a given circuit-interrupting position and is restrained by ratchet wheel 31 against such biased movement pending fusion of the aforementioned solder fllm. The parts aforedescribed are of known form, and the same form no part of the present invention.

As aforestated, we substitute for the usual coiled wire heater element a device of the character now to be described. Our device comprises essentially an outer tubular metal member 49, the outside diameter of which is such as to permil; ready insertion thereof into recess II and between the upstanding arms of the aforementioned metal supporting member IS; an inner tubular metal member 4| (Fig. 3) whose inside diameter is such that it is easily slidable downwardly over the upper end portion 25 of the member forming part of the aforedescribed thermally responsive device. The lower ends of tubular members 40 and 4| are arranged in flush relationship to each other, and the same are rigidly mechanically connected to each other by an interposed annular or disk-like body 42 composed of a mixture of ingredients in powder form molded in situ under relatively high pressure and subjected to a suitable heat treatment as hereinafter described.

We prefer to provide the tubular member 49 with an external peripheral groove 43 adjacent the lower end thereof, thus forming a thinned and more readily deformable wall portion 44, which is bulged outwardly as shown as an incident to molding of the body 42, whereby a positive mechanical interlock is provided between the latter and member 40. Member 4| is likewise provided adjacent its lower end with a plurality of outwardly projecting annular ribs orvanes 45, 49, 41, which initially extend outwardly in planes at substantially right angles to the longitudinal axis of member 4|; but the upper rib 45 is bent or deflected downwardly to a considerable extent, and the intermediate rib 49 is bent downwardly to a slight extent as an incident to application of the necessary molding pressure to the body of material 42. Such deformation of at least some of the ribs 45, 49 and 41 insures a good mechanical interlocking connection between tubular member 4| and the body 42 forming the heating resistor element of the device. The mechanical interlocking connections aflorded between body 42 and each of the tubular members 40 and 4| also insure good electrical and thermal connections between these parts.

Member 4| is preferably provided exteriorly thereof between its upper end and the body 42 with a peripheral groove 48 which is adapted to act as a heat block to minimize the amount of heat transmitted through member 4| to its terminal end.

The members 40 and 4| are provided with exteriorly cut-away or reduced upper end portions d 49 and 50 to respectively accommodate the circular openings and 52 formed in a pair of terminal members 53 and 54; said terminal members being. rigidly and permanently mechanically and electrically connected to the respective members 50 and 8! by soldering (preferably silver soldering), brazing, or welding the same, as indicated at 55 and 56 in Fig. 3. Terminal members 53 and 54 have perforated end portions 51 and 58 bent at right angles thereto to overlie the respective openings (not shown) in a pair of wiring terminal plates 59 and 60 (Fig. 1). Plates 59 and 50 are respectively provided with outer openings (not shown); and the two openings in each of plates 59 and 60 respectively overlie recessed and tapped metal inserts (not shown) in base section to accommodate the shanks of -.a corresponding number of wiring terminal screws 6|. Thus the outer screws 6| provide for electrical connection of the circuit wires to plates 59 and 60 while the inner screws 6| provide for electrical connection of the terminals 53 and 05 to the respective plates 59 and 60.

In the slightly modified form of our invention illustrated in Fig. 4 most of the parts may be identical with those aforedescribed, and the corresponding parts have been given like. numerals of reference. However, the molded and heat sintered body 52 is of substantially less thickness than that shown at 42 in Fig. 3. Therefore, al-

though the member M in Fig. 4- may be me.- chined from the same metal blank or piece as the corresponding member in Fig. 3, we prefer to provide upon the member 8! in Fig. 4 only two peripheral ribs or vanes, which are designated by the numerals 63 and 65; it being understood that the rib 83 is substantially deformed or bent downwardly as an incident to the molding pressure, to afiord the aforementioned good mechanical, electrical and thermal connection between the molded body 62 and member ti Also as shown in Fig. 4, we prefer to coat or enamel the upper and lower surfaces of the molded body 6-2 with a suitable medium, such as blue frit, as indicated at 65 and 66; the coatings or blue frit in their final form acting to completely seal ofi the molded body 52 from contact with the surrounding atmosphere, thus assisting in stabilizing the operating and overload current controlling characteristics of the unit as a whole. It is to be understood that the molded and heat sintered body 42 in the device of Figs. 1 to 3 may likewise be provided with upper and lower protective and sealing layers of blue frit in a fused and hardened condition, as in Fig. 4, with consequent attainment of the desirable results aforementioned. On the other hand, many of the desirable operating characteristics of our device may be attained without the use of the aforementioned enamel coatings, especially in respect of certain uses of the device, and in certain installations where the variability of the operating conditions to be met with is at a minimum.

As a result of extensive experiments, we have found that many different conducting and non- 65 conducting materials and binding agents may be employed in various different proportions, according to the operating characteristics (including resistance values) desired in the article in final form.

Thus, of the powdered metals which may be employed where a positive temperature coemcient of resistance is desired in the resistance element, we prefer to use a metal (or combinations of two or more of the metals) of the group comprising:

(1) iron; (2) nickel; (3) chromium: (4) tungsten; (5) silver; and (6) titanium.

Of the powdered materials which may be employed where a negative temperature coeflicient 5 of resistance is desired, we prefer to employ either: (1) carbon (preferably in the form of graphite), or (2) Carborundum.

As non-conducting ingredients of the resistance element we prefer to employ (in colloidal form) either: (1), bentonite; or (2) kaolin, or a mixture thereof.

As binding agents we prefer to combine with the molding .mixture (when required) either: 1) sodium silicate; (2) chromic acid, or (3) magnesium oxychloride, or a mixture of any two or all of said materials. As will be noted from certain of the examples hereinafter set forth, it may not be necessary to employ any of the aforementioned binding agents, regardless of whether or not the resistance element is to be sintered before use thereof; such omission of the binding agent, of course, depending upon the particular character of the conducting and non-conducting materials employed in producin the element.

When it is desired to sinter the molded resistance element prior to use of the same, we prefer to include in the molding composition a predetermined proportion of a suitable frit or enamel (such as blue frit); although here again such a binder may or may not be required, but depends upon the particular character of the conducting and non-conducting ingredients of the molding mixture.

Whether or not such blue frit is included as a part of a particular molding mixture, we have found it desirable in most instances where the resistance element is to be sintered to provide the otherwise exposed faces of the body thereof with a coating of blue frit to be fused as an incident to the sintering operation. Such blue frit or enamel coating assists in stabilizing the operating characteristics of the element, and seals and strengthens the same.

The following is one method of producing a resistance element in accordance with our invention whereby an extremely low resistance value is provided:

Example I A mixture of ingredients in powder form in the following proportions by weight was employed:

Per cent A mixture of nickel and chromium (of which w After thoroughly mixing said ingredients in a suitable manner the mixture was placed in a humidifier and allowed to remain there for fortyeight hours. We then deposited two grams of the humidified powder mixture into each of a plurality of die cavities, into each of which a set of the tubular metal parts 40 and 4| had previously been properly positioned. The parts of the molding dies were then heated to 400 degrees F., and then operated to effect application of a '10 pressure of 10,000 pounds per square inch to the powder mixture.

The articles consisting of the molded bodies (42) and the respective sets of tubular metal members 40 and ll united thereby were then removed from the molding cavities and sintered at sistance value of .000576 ohm. Such unit was:-

then assembled with respect to a thermally responsive device like that shown in Fig. 3, and upon subjection of the circuit containing the same to a current of 80 amperes, the tripping time was found to be 5.22 minutes (under a condition of 40 degrees C. ambient temperature). Under the same ambient temperature condition, upon subjection of the circuit to a current of 78 amperes, the tripping time was found to be 7.15 minutes. Under said ambient temperature condition when the circuit was subjected to a currentof 77 amperes the thermal overload would not trip during a period of 30 minutes. Therefore, assuming that said device would require a current of at least 77.5 amperes to effect tripping thereof, the rating of the unit would be: 77.5 times 1.05, equals 81.4. The device when subjected to an overload of 1000 per cent for a number of times performed satisfactorily.

Example II A mixture of ingredients in powder form in the following proportions by weight was employed:

Per cent Iron 33 Carborundum 47 Kaolin 10 Sodium silicate 10 After mixing and molding the powdered ingredients and sintering the molded articles in the manner set forth in Example 1, except that 3 grams of the powdered mixture were employed for each article, one of the same, tested for an example, was found to have a resistance value of .017 ohm. The tripping current for said unit was found to be 30 amperes, and the same withstood a number of 1000 per cent overloads very well. This sample was also subjected to the four times fusing 5000 ampere short circuit test. The fuse burned out, and no fire hazard whatsoever resulted from operation of this resistance heater unit. On the contrary, the resistance element was found to remain almost at room temperature. Such a result would, of course, notbe possible with a wire coil heater of like or similar operating character or rating.

Example III A mixture of ingredients in powder form in the following proportions by weight was employed:.

Per cent Iron 35 Carborundum Kaolin 10 Blue frit 10 The units were then baked at 1200 degrees I". for about one hour. The temperature was then raised to. 1500 degrees F., whereupon the blue frlt fused. The supply of heatv to the furnace was then discontinued. and the units were removed after the blue frit enamel had hardened. One of the completed units, having a measured resistance value of 2.89 ohms, was tested as follows in conjunction with a conventional overload device (such as that shown in Figs. 1, 2 and 3) with a resultant slight variation in resistance value:

The resistance was measured at the end of each three tripping operations, and it will be noted that the resistance variation amounted to only 0.1 ohm throughout the entire range of currents to which the unit was subjected.

Example IV A mixture of ingredients in powder form in the following proportions by weight was employed:

Percent Iron 38 Carborundum 42 Kaolin 20 Example 7 A mixture of ingredients in powder form in the following proportions by weight was employed:

Percent Carborundum 45 Kaolin 20 Said ingredients were mixed, molded and heat sintered in the manner set forth in Example IV. The units were enameled with a low melting point frlt. Units produced from the materials and in the manner above set forth were found to have a resistance value of approximately .06 ohm.

Although we have herein specifically mentioned a molding pressure of 10,000 pounds per square inch, it is to be understood that the molding pressure employed may be varied throughout a very wide range. according to the physical characteristics of the particular ingredients of the resistance composition employed and/or the particular characteristics desired in the final article. For example. we have in some cases employed molding pressures as high as twenty-five tons per 76 squareinch.

Also while we have herein set forth only certain examples of methods by which the articles herein contemplated may be produced, it is to be understood that innumerable other combinations and proportions of ingredients, and the steps of treating the same, may be employed; but in all cases the advantage of non-fusibility of the resistance element under excessive overload conditions will be attained, as well as other advantages hereinabove set-forth.

We claim:

1. In a device of the character described, in combination, an outer tubular metal member, an inner tubular metal member arranged in concentric re ationship thereto, and an integral mass of 'molded powder composition resistance material interposed between said members and rigidly and permanently attached thereto throughout the inner periphery of the outer tubular memher and throughout the outer periphery of the inner tubular member, said resistance material comprising essentially a conducting material and a non-conducting material in predetermined proportions, and said resistance material being characterizedby its ability to withstand extremely high overload currents passing therethrough without fusing.

2. In a device of the character described, in combination, an outer tubular metal member of circular form in transverse cross section, an inner tubular metal member of circular form in transverse cross section arranged in concentric relationship thereto, an integral mass of molded powder composition resistance material interposed between said members and rigidly and permanently attached thereto throughout the inner and outer peripheries thereof respectively, said resistance material comprising essentially aconducting material and a non-conducting material in predetermined proportions, said resistance material being characterized by its ability to withstand without fusing overload currents which are extremely high with respect to the normal current rating of the device, and a pair of metal terminal members rigidly and permanently attached to the respective tubular members and acting as a support for the device as a whole.

3. A resistance heater unit for thermal overload devices, comprising a pair of concentric outer and inner tubular metal members, said members being rigidly and permanently mechanically and electrically connected with each other by an integral annular body composed of a mixture of conducting and non-conducting materials in powder form molded under relatively high pressure in situ therebetween, and-said materials being so proportioned relatively to each other as to afford a predetermined value of resistance of said molded body.

4. A resistance heater unit for thermal overload devices, comprising a pair of concentric outer and inner tubular metal members, said members being rigidly and permanently mechanically and elctrically connected with each other by an integral annular body composed of a mixture of conducting and non-conducting materials in powder form molded under relatively high pressure in situ therebetween, said materials being so proportioned relatively to each other as to afford a predetermined value of resistance of said molded body, and a pair of punched and stamped sheet metal terminal members rigidly and permanently mechanically and electrically connected to the respective tubular members to afiord i0 mechanical support and electrical connections for the unit as a whole.'

5. A resistance heater unit for thermal overload devices, comprising a pair of concentric outer and inner tubular metal members of circular form in transverse cross section, said members being rigidly and permanently mechanically and electrically connected with each other by an integral annular body composed of a mixture of conducting and non-conducting materials in powder form molded under relatively high pressure in situ therebetween, said materials being so proportioned relatively to each other as to afiord a predetermined value of resistance of saidmolded body, said conducting material consisting of one or more substances of the group composed of iron nickel chromium, tungsten, silver, titanium, carbon and Carborundum, and said hon-conducting material consisting of one or more substances of the group composed of bentonite and kaolin.

6. A resistance heater unit for thermal overload devices, comprising a pair of concentric outer and inner tubular metal members of circular form in transverse cross section, said members being rigidly and permanently mechanically and electrically connected with each other solely by an integral annular body composed of a mixture of conducting and non-conducting materials in powderform molded under relatively high pressure in situ therebetween, said materials being so proportioned relatively to each other as to afford a predetermined value of resistance of said molded body, said conducting material consisting of one or more metals of the group composed of iron, nickel, chromium, tungsten, silver and titanium, and said non-conducting material consisting of one or more substances of the group composed of bentonite and kaolin.

7. A heater unit for thermal overload devices, comprising a pair of concentric outer and inner tubular metal members, said members being rigidly and permanently mechanically and electrically connected with each other solely by an integral annular body composed of a mixture of conducting and non-conducting materials in powder form molded under relatively high pressure in situ therebetween, a portion of said nonconducting material being composed of a binding agent selected from the group consisting of sodium silicate, chromic acid and magnesium oxychloride, and said conducting and non-conducting materials being so proportioned relatively to each other as to aiTord a predetermined value of resistance of said body.

8. A heat sintered heater unit for thermal overload devices, comprising a pair of concentric outer and inner tubular metal members of circular form in transverse cross section, said members being rigidly and permanently mechanically and electrically connected with each other solely by an integral annular body composed of a mixture of conducting and non-conducting materials in powder form molded under relatively high pressure in situ therebetween, said mixture having incorporated therein a binding agent selected from the group consisting of sodium silicate, chromic acid and magnesium oxychloride, said molded mixture when subjected to a predetermined heat sintering treatment being adapted to provide a predetermined value of resistance of said body, and a pair of punched and stamped sheet metal terminal members rigidly and permanently mechanically and electrically connected to the respective tubular members to afford mechanical support and electrical connections for the unit as a whole.

9. A heat sintered resistance heater unit for thermal overload devices, comprising a pair of outer and inner tubular metal members of circular form in transverse cross section, said members oeing rigidly and permanently mechanically and electrically connected with each other solely by an integral annular body composed of a mixture oi conducting and non-conducting materials in powder form molded under relatively high pressure in situ therebetween, said conducting material consisting of one or more substances of the group composed of iron, nickel, chromium, tungsten, silver, titanium, carbon and Carborundum, said non-conducting material consisting of one or more substances of the group composed of bentonite and kaolin, said mixture having incorporated therein a binding agent selected from the group consisting of sodium silicate, chromic acid and magnesium oxychloride, and the ingredients of said mixture being so proportioned relatively to each other as to provide'a predetermined value of resistance of the molded body after a predetermined heat sintering treatment thereof.

10. A heat sintered resistance heater unit for thermal overload devices, comprising a pair of outer and inner tubular metal members of circular form in transverse cross section, said members being rigidly and permanently mechanically and electrically connected with each other solely by an integral annular body composed of a mixture of conducting and non-conducting materials in powder form molded under relatively high pressure in situ therebetween, said conducting material consisting of one or more metals oi the group composed of iron, nickel, chromium, tungsten, silver and titanium, said non-conducting material consisting of one or more substances of the group composed of bentonite and kaolin,'said mixture having incorporated therein a binding agent selected from the group consisting oi sodium silicate, chromic acid and ma nesium oxychloride, and the ingredients oi said mixture being so proportioned as to provide a predetermined valueof resistance of the molded body after a predetermined degree of heat sintering treatment thereof.

11. A heat sintered resistance heater unit for thermal overload devices, comprising a pair oi outer and inner tubular metal members oi circular form in traverse cross section, said members being rigidly and permanently mechanically and electrically connected with each other solely by an integral annular body composed a mixture oi conducting and non-conducting materials in powder form molded under relatively high pressure in situ therebetween, said conducting material consisting of one or more metals of the group composed of iron, nickel, chromium, tungsten, silver and titanium. said non-conducting material consisting oi one or more substance oi the group composed oi bentonite and kaolin. said mixture having incorporated therein a bindingagent selected from the group consisting of sodium silicate, chromic acid and magnesium 0X!- chloride, the ingredients oi said mixture being so proportioned as to provide a predetermined value or resistance or the molded body alter a predetermined degree of heat'sintering treatment thereof. and the exposed surfaces oi said molded body having a coating 0! blue irit applied thereon and rigidly united therewith, as an incident to 12 to completely seal said body from contact 1 ith the atmosphere.

12. A heat sintered heater unit for thermal overload devices, comprising a pair of concentric outer and inner tubular metal members of circular form in transverse cross section, said members being rigidly and permanently mechanically and electrically connected with each other solely by an integral annular body composed or a mixture of conducting and non-conducting materials in powder form molded under relatively high pressure in situ therebetween, said materials consisting of 33 per cent by weight of iron, 4'7 per cent oi Carborundum, 10 per cent of kaolin, and 10 per cent of sodium silicate, said molded body being characterized by a resistance value of 2214 ohms when sintered at 1800 degrees F. for a period of fifteen minutes and the same being further characterized by a resistance value of 19.8 ohms when sintered at the aforementioned temperature for a period of ninety minutes.

13. A resistance heater unit for thermal overload devices, comprising a pair of concentric outer and inner tubular metal members, said members being rigidly and permanently mechanically and electrically connected with each other solely by an integral annular body composed of a mixture of conducting and non-conducting materials in powder form molded under relatively high pressure in situ therebetween, said materials being so proportioned relatively to each other as to ail'ord a predetermined value of resistance of said molded body when heat sintered at a predetermined relatively high temperature ior a predetermined length of time, and said molded body being characterized by a variation in its resistance value substantially inversely proportional to an increase in the length of the sintering treatment at said temperature.

14. The method of making a resistance heater unit for a thermal overload device, which comprises rigidly and permanently mechanically and electrically connecting a pair oi outer and inner tubular metal members solely by molding under relatively heavy pressure in situ therebetween an integral annular body composed oi a mixture of conducting and non-conducting materials in powder form, said materials being so proportioned relatively to each other as to afford a predetermined value of resistance oi said molded body.

15. The method of making a resistance heater unit for a thermal overload device, which comprises rigidly and permanently mechanically and electrically'connecting a pair of outer and inner tubular metal members solely by molding under relatively heavy pressure in situ therebetween an integral annular body composed oi a mixture oi conducting and non-conducting materials in powder form, said materials being so proportioned relatively to each other as to afford a predetermined value of resistance of said molded body, and thereafter rigidly and permanently mechanically and electrically connecting to the respective tubular members a pair of punched and stamped sheet metal terminal members to afford mechanical support and electrical connections for the unit as a whole.

16. The method of making a resistance heater unit for thermal overload devices, which comprises rigidly and permanently mechanically and electrically connecting a pair oi outer and inner tubular metal members solely by molding in situ therebetween under a pressure oi at least ten said heat sintering treatment, said coating actins 1s thousand pounds per square inch an integral annular body composed of a mixture of conducting and non-conducting materials in powder form, said conducting material consisting of one or more substances of the group composed of iron, nickel, chromium, tungsten, silver, titanium, carbon and Carborundum, and said non-conducting material consisting of one or more substances of the group composed of bentonite and kaolin, said materials being so proportioned relatively to each other as to aiford a predetermined value of resistance of said molded body.

17. The method of making a resistance heater unit for thermal overload devices, which comprises rigidly and permanently mechanically and electrically connecting a pair of outer and inner tubular metal members solely by molding in situ therebetween under a pressure of at least ten thousand pounds per square inch an integral annular body composed of a mixture of conducting and non-conducting materials in powder form, said conducting material consisting of one or more metals of the group composed of iron, nickel, chromium, tungsten, silver and titanium, said non-conducting material consisting of one or more substances of the, roup composed of bentonite and kaolin, said materials being so proportioned relatively to each other as to aflord a predetermined value of resistance of said molded body, and then subjecting said molded body to a heat sintering treatment at a predetermined relatively high temperature for a predetermined period of time to efiect a predetermined variation in said resistance value.

18. The method of making a resistance heater unit for thermal overload devices, which comprises rigidly and permanently mechanically and electrically connecting a pair of outer and inner tubular metal members of circular form in transverse cross section solely by molding in situ there- Patent Nb. 2,479,914

LESTER D. DRUGMAND ET AL.

14 between under a pressure of approximately ten thousand pounds per square inch an annular integral body composed of a mixture of predetermined proportions of conducting and non-conducting materials in powder form, said mixture having incorporated therein a binding agent selected from the group consisting of sodium silicate, chromic acid and magnesium oxychioride, said conducting material consisting of one or more metals of the group composed of iron, nickel, chromium, tungsten, silver and titanium, said non-conducting material consisting of one or more substances of the group composed of bentonite and kaolin, subjecting said molded mixture to sintering at a predetermined temperature for a predetermined period of time to provide a predetermined value of resistance of said body, and thereafter rigidly and permanently mechanically and electrically connecting to the respective tubular members a pair of punched and stamped sheet metal terminal members to afford mechanical and electrical connections for the unit as a whole.

LESTER D. DRUGMAND. EDWIN W. SEEGER.

REFERENCES CITED The following references are of record in the file of this patent:

UNITED STATES PATENTS Durham Dec. 3, 1935 Certificate of Correction August 23, 1949 It is hereby certified that errors appear in the printed specification of the above numbered patent requ1r1ng correct on as follows:

Column 9, line 65, for elctrically for annular integral after line 36, insert the following:

Slepian read electrically; column 14, lines 2 and 3, read 'mtegral annular; same column, list of references cited,

June 10, 1930 and that the said Letters Patent should be read with these corrections therein that the same may conform to the record of Signed and sealed this 7th day of the case in the Patent Office. February, A. D.-1950.

THOMAS F. MURPHY, 

